1. Field of the Invention
The present invention relates to a thin film magnetic memory device, and more particularly relates to a random access memory provided with memory cells having magnetic tunnel junctions (MTJs).
2. Description of the Background Art
As a memory device capable of storing nonvolatile data with low consumption power, attention has been paid to an MRAM (Magnetic Random Access Memory) device. The MRAM device is a memory device which stores nonvolatile data using a plurality of thin film magnetic elements formed on a semiconductor integrated circuit and which can access each of the thin film magnetic elements.
Recently, it has been made public that the performance of the MRAM device surprisingly advances by using tunneling magneto-resistance element which are thin film magnetic bodies using magnetic tunnel junctions (MTJ's) as memory cells. The MRAM device provided with memory cells having MTJs is disclosed by technical documents such as “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”, ISSCC Digest of Technical Papers, TA7.2, February 2000., “Nonvolatile RAM based on Magnetic Tunnel Junction Elements”, ISSCC Digest of Technical Papers, TA7.3, February 2000., and “A 256 Kb 3.0V 1T1MTJ Nonvolatile Magnetoresistive RAM”, ISSCC Digest of Technical Papers, TA7.6, February 2001.
FIG. 22 is a schematic view showing configuration of a memory cell having a magnetic tunnel junction (hereinafter, also referred to simply as “MTJ memory cell”).
Referring to FIG. 22, the MTJ memory cell is provided with a tunneling magneto-resistance element TMR in which electric resistance changes with respect to stored data level, and an access element ATR for forming a path of a sense current Is passing through tunneling magneto-resistance element TMR during data read. Since access element ATR is typically formed of a field effect transistor, access element ATR will be also referred to as “access transistor ATR” hereinafter. Access transistor ATR is connected in series to tunneling magneto-resistance element TMR.
A write word line WWL for indicating data write, a read word line RWL for executing data read, and a bit line BL which is a data line for transmitting an electrical signal corresponding to the data level of stored data during the data read and data write, are arranged for the MTJ memory cell.
FIG. 23 is a conceptual view for explaining a data read operation from the MTJ memory cell.
Referring to FIG. 23, tunneling magneto-resistance element TMR includes a ferromagnetic layer (hereinafter, also referred to as “fixed magnetic layer”) FL having a fixed, constant magnetization direction, and a ferromagnetic layer (hereinafter, also referred to as “free magnetic layer”) VL magnetized in a direction according to a magnetic field applied externally. A tunneling barrier (tunneling film) TB formed of an insulating film is provided between fixed magnetic layer FL and free magnetic layer VL. Free magnetic layer VL is magnetized in the same direction as or the opposite direction to that of fixed magnetic layer FL in accordance with the data level of stored data to be written. Fixed magnetic layer FL, tunnel barrier TB and free magnetic layer VL form a magnetic tunnel junction.
In the data read operations, access transistor ATR is turned on in response to the activation of read word line RWL and tunneling magneto-resistance element TMR is connected between bit line BL and a ground voltage Vss. As a result, a bias voltage in accordance with the voltage of the bit line is applied to the both ends of tunneling magneto-resistance element TMR and a tunnel current flows in tunneling film (tunneling barrier) TB. By using such a tunnel current, it is possible to carry a sense current to the current path formed of bit line BL, tunneling magneto-resistance element TMR, access transistor ATR, and ground voltage Vss.
The electric resistance of tunneling magneto-resistance element TMR changes according to the relative relationship between the magnetization direction of fixed magnetic layer FL and that of free magnetic layer VL. Specifically, if the magnetization direction of fixed magnetic layer FL is same (parallel) to that of free magnetic layer VL, the electric resistance value of tunneling magneto-resistance element TMR is a minimum value Rmin, and if these magnetization directions are opposite (non-parallel) to each other, the electric resistance value of tunneling magneto-resistance element TMR is a maximum value Rmax.
Accordingly, if free magnetic layer VL is magnetized in a direction according to the stored data, a voltage change which occurs to tunneling magneto-resistance element TMR due to sense current Is differs according to the level of the stored data. Therefore, if sense current Is is carried to tunneling magneto-resistance element TMR after precharging bit line BL with a constant voltage, for example, the stored data of the memory cell can be read by sensing the voltage of bit line BL.
FIG. 24 is a conceptual view for explaining a data write operation to the MTJ memory cell.
Referring to FIG. 24, during data write, read word line RWL is inactivated and access transistor ATR is turned off. In this state, a data write current for magnetizing free magnetic layer VL in a direction according to the write data, is carried to each of write word line WWL and bit line BL.
FIG. 25 is a conceptual view for explaining the relationship between the data write current and the magnetization direction of tunneling magneto-resistance element TMR during data write.
Referring to FIG. 25, the horizontal axis indicates a magnetic field applied in an easy axis (EA) direction in free magnetic layer VL in tunneling magneto-resistance element TMR. On the other hand, the vertical axis H (HA) indicates a magnetic field effecting in a hard axis (HA) direction in free magnetic layer VL. Magnetic fields H (EA) and H (HA) correspond to two magnetic fields generated by currents carried to bit line BL and write word line WWL, respectively.
In the MTJ memory cell, the fixed magnetization direction of fixed magnetic layer FL is along the easy axis of free magnetic layer VL, and free magnetic layer VL is magnetized in a direction parallel or non-parallel (opposite) to fixed magnetic layer FL along the easy axis direction in accordance with the level of stored data (“1” and “0”). The MTJ memory cell can store 1-bit data (“1” and “0”) corresponding to the two magnetization directions of free magnetic layer VL, respectively.
The magnetization direction of free magnetic layer VL can be rewritten only if the sum of magnetic fields H (EA) and H (HA) applied to free magnetic layer VL reaches a region outside of an asteroid characteristic line shown in FIG. 25. In other words, if the data write magnetic field applied to free magnetic layer VL has an intensity corresponding to the region inside of the asteroid characteristic line, the magnetization direction of free magnetic layer VL does not change.
As shown in the asteroid characteristic line, if a magnetic field in the hard axis direction is applied to free magnetic layer VL, it is possible to decrease a magnetization threshold value necessary to change the magnetization direction of free magnetic layer VL along the easy axis.
If operation points during data write are designed as shown in the example of FIG. 25, the data write magnetic field in the easy axis direction is designed so as to have an intensity of HWR in the MTJ memory cell to which the data is to be written. That is, the value of the data write current carried to either of bit line BL or write word line WWL is designed so as to obtain data write magnetic field HWR. Generally, data write magnetic field HWR is expressed by the sum of a switching magnetic field HSW necessary to change over a magnetization direction and a margin ΔH, i.e., HWR=HSW+ΔH.
To rewrite the stored data of the MTJ memory cell, i.e., to rewrite the magnetization direction of tunneling magneto-resistance element TMR, it is necessary to carry a data write current at predetermined level or more to each of write word line WWL and bit line BL. Thus, free magnetic layer VL in tunneling magneto-resistance element TMR is magnetized in the direction parallel or opposite (non-parallel) to that of fixed magnetic layer FL in accordance with the direction of the data write magnetic field along the easy axis (EA). The magnetization direction which is written to tunneling magneto-resistance element TMR once, i.e., the stored data of the MTJ memory cell held in a nonvolatile manner until new data is written.
As described above, the electric resistance of tunneling magneto-resistance element TMR changes according to the magnetization direction which can be rewritten by the data write magnetic field applied thereto, so that if electric resistance values Rmax and Rmin of tunneling magneto-resistance element TMR are made to correspond to the levels (“1” and “0”) of the stored data, it is possible to store data in a nonvolatile manner.
Thus, sense current Is which passes through tunneling magneto-resistance element TMR during data write is carried as the tunnel current which passes through tunneling film (tunneling barrier) TB. However, the voltage-to-current characteristic, i.e., voltage applied to tunneling film (bias voltage) to tunnel current characteristic of tunneling magneto-resistance element TMR largely depend on the thickness of the tunneling film. Therefore, depending on the manufacturing irregularity of the tunneling film thickness in a manufacturing process, sense current Is during data write largely changes.
In other words, even if the same bias voltage is applied, sense current Is carried to tunneling magneto-resistance element TMR largely changes depending on the manufacturing irregularity of the tunneling film thickness, so that the electric resistance value of tunneling magneto-resistance element TMR, i.e., the level of stored data cannot be accurately sensed from the voltage of the bit line. It is, therefore, necessary to constitute the MTJ memory cell so as to be capable of securing a data read margin corresponding to such manufacturing irregularity.
In addition, the electric resistance characteristic of tunneling magneto-resistance element TMR largely depends on temperature and bias voltage, whereby it is also necessary to consider securing the data read margin to correspond to these variations.
On the other hand, the reliability of the tunneling film largely depends on tunnel current. In other words, if a thin tunneling film is manufactured due to manufacturing irregularity, there is a probability that an excessive tunnel current flows during an ordinary operation to disadvantageously deteriorate the operation reliability of the entire MRAM device.
Furthermore, while it is necessary to conduct a defect acceleration test to ensure the reliability of the tunneling film so as to evaluate the reliability of tunneling magneto-resistance element TMR, tunneling film screening cannot be effectively carried out in an acceleration test conducted applying a high electric field as executed for a conventional MOS (Metal Oxide Semiconductor) type LSI (Large Scale Integrated Circuit).